Abstract

The development of high-energy lasers requires optical windows capable of handling megajoule beam energies without compromising the system’s performance. Calcium fluoride has been identified as a prime candidate for windows operating at chemical laser wavelengths due to very low bulk absorption and exceptionally small thermal lensing coefficients; it is, however, vulnerable to structural failure owing to poor mechanical strength characteristics and a large thermal stress factor. It is, therefore, essential to properly assess the ultimate potential of this material, which we attempt to do here in the following manner: (a) We assemble reliable numbers for all pertinent properties of (111)-oriented single crystals and polycrystalline isotropic aggregates (PIAs), such as fusion-cast , which requires addressing issues relating to the elastic properties, the stress-optic coefficients, and the flexural strength. (b) We provide correct analytical expressions for evaluating the impact of pressure- and beam-induced effects on wave-front phase distortions and mechanical failure modes, taking advantage of a previous investigation [J. Appl. Phys.98, 043103 (2005)]. (c) We perform detailed calculations on “model” windows made of either or that transmit optimally truncated Gaussian beams at wavelengths of 1.15 and , for run times such that lateral heat conduction and surface cooling can be ignored. Our main conlusions are as follows: (a) With windows thermal lensing, as measured in terms of the Strehl ratio and on assuming coating absorptances of no more than , is of no consequence in the sense that catastrophic failure may occur at fluence levels way below the threshold for optical distortion. (b) Evidence of a poor Weibull shape factor degrades the design safety margins, which requires operating at peak intensities of no more than to achieve optimum on-target fluences. (c) Regarding the issue of vs , we note that fusion-cast material outperforms single crystals based on the figure of merit for distortion, as well as fracture and yield strengths, but contrary to (111)-oriented material, it exhibits birefringence that may rule out its use if depolarization is of concern.